(i) PEPTIDE LINKAGE vs GLYCOSIDIC LINKAGE

PEPTIDE LINKAGE
- It is an AMIDE linkage, $-CO-NH-$
- It is formed between the $-COOH$ group of one alpha-amino acid and the $-NH_2$ group
of another alpha-amino acid, with the ELIMINATION OF A WATER MOLECULE.
$$H_2N-CHR-COOH + H_2N-CHR'-COOH \rightarrow H_2N-CHR-\underset{\text{peptide bond}}{CO-NH}-CHR'-COOH + H_2O$$
- It is found in PEPTIDES and PROTEINS.

GLYCOSIDIC LINKAGE
- It is an ETHER-type linkage, $-O-$
- It is formed between two MONOSACCHARIDE units through an OXYGEN atom, again with the
ELIMINATION OF A WATER MOLECULE (between the anomeric $-OH$ of one sugar and an $-OH$
of the other).
- It is found in DISACCHARIDES and POLYSACCHARIDES, e.g. sucrose, maltose, starch,
cellulose.

(ii) ESSENTIAL vs NON-ESSENTIAL AMINO ACIDS

ESSENTIAL AMINO ACIDS
- These CANNOT be synthesised in the human body.
- They must be obtained ready-made through the DIET (protein food).
- Examples: valine, leucine, isoleucine, lysine, phenylalanine, threonine, methionine,
tryptophan.

NON-ESSENTIAL AMINO ACIDS
- These CAN be synthesised in the human body itself.
- They need not be supplied through the diet.
- Examples: glycine, alanine, serine, aspartic acid, glutamic acid.